How do caves form?

The most common caves, called solution caves, form in limestone and similar rocks. Rainwater absorbs carbon dioxide from the air and soil, becoming a weak acid that slowly dissolves the rock as it seeps through cracks. Over very long periods this widens the cracks into passages and chambers, sometimes vast ones. Inside, dripping mineral-rich water builds features like stalactites hanging from the ceiling and stalagmites rising from the floor. Other caves form in different ways, such as lava tubes left by flowing molten rock, or sea caves carved by pounding waves.